Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. The objectives of RFID systems are to design a reliable, secure, novel architecture and to minimize the total cost of the Interrogator and the Tags, while meeting the system performance requirements. In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. Most commonly, this communication utilizes Time-Division Duplex (TDD) or Half Duplex techniques. Full Duplex (FD) techniques are also possible, as disclosed in pending U.S. patent application Ser. No. 08/492,174, and are particularly useful where a fast response is needed. In a TDD system, after transmitting a message to the Tag, the Interrogator then transmits a Continuous-Wave (CW) radio signal to the Tag. The Tag then modulates the CW signal using Modulated BackScattering (MBS) where the antenna is electrically switched, by the modulating signal, from an "open" to a "short".
Examples of prior art in MBS technology are described in U.S. Pat. Nos. 4,075,632 and 4,360,810. MBS systems typically utilize the amplitude-modulated techniques described above for communications from the Interrogator to the Tag, and utilize MBS for communications from the Tag to the Interrogator. The MBS communications can utilize any conventional modulation technique. MBS systems are desired to operate successfully in the presence of multiple Tags in the Interrogator reading field. There are classically known techniques (e.g., Simple Aloha, Slotted Aloha) which allow multiple devices to respond to a central communications device when each of those devices is not aware of the presence of the others. This invention discloses improved methods for MBS systems to successfully operate with multiple Tags responding in an independent and simultaneous manner.